I've read lots of stuff that supports this. We kill a lot of beneficial microorganisms with antibiotics and radiation and over sterilization. Some produce, for example, is irradiated to kill off microorganisms to extend its shelf life. If you're not a fan of that, buy organic, or add fermented food to your diet, like yogurt, kefir, miso, kimchi, sauerkraut, etc., especially if you are taking or recently took antibiotics.